The idea that we should behave towards others in the way we would like to be treated by them is an old one. It appears in different philosophies and when Jesus said it, he wasn't saying anything new to his Jewish followers. The same words appear very early in what we call the Old Testament, the Hebrew Bible. What was new was that Jesus put the idea right at the front of all other ideas of how to act towards other people. He told his followers that they must love and worship God above all else, then he added this simple rule.
He told his disciples that no matter what else they found in scripture, all other commandments flowed from loving your neighbour as yourself. If we look at the famous Ten Commandments, we can easily see that the ones talking about behaviour all come from that one rule for life. We may not 'covet our neighbour's ox or donkey' in these times, but the reason for saying that is that we would not wish anyone to covet anything that we thought of as our own.
What Jesus was doing was sweeping aside a lot of human-written rules that had built up over the centuries as being 'God's laws' when to Jesus they were invented rituals, such as food prohibitions and rules on what you could or could not wear. To Jesus those other rules were just getting in the way of two things: your correct relationship with God and your correct relationship with other people.
It was completely revolutionary in its time and you can see why the authorities, Jewish and Roman, were frightened of what he was saying. If rules and laws were to be done away with and everyone was to decide how to behave based on whether they doing as they would like to be done to, what might people really do? What if someone said, 'Well, I am big and strong and want to try to take stuff from my neighbour; I don't care if they treat me the same because I can defend myself. So I am just going to take anything that I want from anyone.' More importantly, what if they decided not to obey officials or pay taxes or give money to the Temple? The authorities might lose that authority, lose their power and soon have nothing.
The same would apply nowadays. When a 'peace protester' stands outside the UK Ministry of Defence to object to nuclear weapons or appears in Red Square, Moscow to protest against Russian attacks in Ukraine, they are acting out the rule. They are saying, 'I would not wish to be treated like this, threatened or under attack, and I don't want my neighbours to be treated like this, even if they live thousands of miles away and I have never seen them.' The reaction of the authorities might be at different levels but it is the same: brutal repression at the time of the protest and then changing laws to make protests illegal and subject to fines and imprisonment. Authority cannot accept such challenges.
In Jesus's case, that led directly to his being viciously and publicly slaughtered as an example to others.
The problem for the authorities is that, within forty years, the Temple in Jerusalem had been destroyed and it has never been rebuilt. The Jewish people were scattered and have only recently returned to Israel, where they remain under constant threat from their neighbours. As for the Romans, the Christian faith took over their empire until the empire disappeared. Christianity remained; it still remains and it is growing. The rules that Jesus gave us are still there and we are still quoting them and many of us try hard to follow them. May that number grow. Amen.